function res = mtimes(a,b)
if a.adjoint
    res = adjDz(b);
else
    res = b(:,:,[2:end,end]) - b;
end

function y = adjDz(x)
y= x(:,:,[1,1:end-1]) - x;
y(:,:,1) = -x(:,:,1);
y(:,:,end) = x(:,:,end-1);